Focusing a laser beam

ABSTRACT

A device for focusing a laser beam includes a first telescopic configuration and a second telescopic configuration. The first telescopic configuration includes a collimating optical element for collimating the laser beam and a downstream focusing optical element for focusing the laser beam onto a focal point. The second telescopic configuration includes a first lens and a downstream second lens disposed in the divergent beam path upstream of the first telescopic configuration. The first and second lenses of the second telescopic configuration are moveable relative to each other in the beam direction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/240,658, filed Sep. 29, 2005, now, U.S. Pat. No.7,339,750,whichclaims priority under 35 U.S.C. §119(a) to European Patent ApplicationNo. 04 023 319.9, filed Sep. 30, 2004, the entire contents of which arehereby incorporated by reference.

TECHNICAL FIELD

The application relates to a device for focusing a laser beam.

BACKGROUND

A conventional scanner processing head for fast spatial processing ofworkpieces using laser beams consists of scanner optics usuallyincluding two mirrors that deflect a collimated or almost collimatedbeam in a first and a second spatial direction and guide it onto theworkpiece using a plane field objective. In an alternative system,downstream of the focusing lens, a focused beam is guided by at leastone scanner mirror onto the workpiece. For processing three-dimensionalobjects using such a scanner optics, the position of the focal point ofthe laser radiation in the beam direction must additionally beadjustable. This necessitates a device for focusing the laser beam thatpermits displacement of the focal position in the beam direction.

If the laser connected to the scanner optics is a solid-state laser, theoutput laser beam is preferably transmitted to the scanner opticsthrough an optical fiber (laser light guide). In the scanner optics, thelaser beam emerges from the laser light guide in a diverging manner, iscollimated using a collimation lens, is subsequently deflected by thescanner mirrors and is focused onto the workpiece using a plane fieldobjective. To adjust the focus along the beam axis, the divergence ofthe beam impinging on the focusing objective is changed by, for example,moving the collimation lens along the beam axis.

One disadvantage of this approach is that the collimating lens isrelatively large and therefore heavy, and the displacement of the lensmust be relatively large to obtain a noticeable focus shift of theworking beam. With this approach, the usually required high dynamics ofthe shifting motion involves a very high driving power, requiring alarge and heavy construction (with even increased moving mass) andcomplex control and cooling of the actuators. Moreover, the large andhighly accelerated mass exerts a considerable reaction force on thestatic part of the assembly, which may cause undesired motion oroscillations of the beam guiding system (which is mounted for example,to a robot arm) that can be compensated for or minimized only withconsiderable additional expense.

The documents JP05002146, JP2000197984, JP07116869 and JP2003012346moreover disclose the use of a telescope system having two lenses thatmay have different sizes, to influence the divergence of the beam. Oneof the two lenses is displaced. Typically, the smaller lens that can bedisplaced with less force to change the separation between the twolenses is the lens that is displaced. These lenses are situated in thecollimated beam and not in the divergent beam.

SUMMARY

A device is described for focusing a laser beam and the device permitsdisplacement of the focal position in the beam direction. Using thisdevice, the focal point can be rapidly positioned in the beam direction,thereby preventing, in particular, undesired oscillations of the device.

In some aspects, the device includes a second telescopic configurationincluding a first lens and a downstream second lens, in the divergentbeam path upstream of a first telescopic configuration. The firsttelescopic configuration includes a collimating optical element and adownstream focusing optical element that focuses the collimated laserbeam on a focal point. The first lens and/or the second lens of thesecond telescopic configuration can be displaced relative to each otherin the beam direction. Due to the position of the second telescopicconfiguration in the divergent beam path upstream of the firsttelescopic configuration, the lenses used for the second telescopicconfiguration may have a smaller focal length than the optical elementsof the first telescopic configuration. The lens displacement required toadjust the focus along the beam axis is therefore reduced in a ratio ofthe focal lengths.

In one implementation, the lens diameters of the second telescopicconfiguration are smaller than the diameters of the optical elements ofthe first telescopic configuration. The optical elements of the firsttelescopic configuration may be, for example, lenses or mirrors. Thelenses of the second telescopic configuration are positioned in thedivergent laser beam in such a manner that these lenses may have only asmall diameter and therefore little mass, permitting highly dynamicmotion thereof. The telescopic lenses should basically have a minimumsize to optimize their dynamic properties.

In a further implementation, the first lens and the second lens of thesecond telescopic configuration each have approximately the sameabsolute value of focal length. In this case, motion of both lenses inthe same direction (caused by external forces) only has a very smallinfluence on the focal position.

In one implementation, the second telescopic configuration is disposedin the vicinity of the exit plane of a laser light guide. It is therebypossible to use lenses with particularly small diameters in the secondtelescopic configuration.

In one implementation, the separation between the first lens and thesecond lens of the second telescopic configuration is substantiallyequal to the sum of the absolute values of their focal lengths. The lensseparation is preferably selected in such a manner that themagnification of the laser light guide exit plane on the workpieceremains largely constant when the lenses are displaced. To meet theserequirements, the optimum lens separation is approximately the sum ofthe focal lengths of the lenses.

In a further implementation, the first lens and the second lens of thesecond telescopic configuration are designed as convergent lenses. Inthis case, the telescopic configuration is designed as Kepleriantelescope. This configuration can prevent collision between the beampath and a compact coaxial drive for displacing the lenses. In theKeplerian telescope, an intermediate focus forms after the second lenssuch that the diameter of the beam at an axial distance of approximatelytwo focal lengths downstream of the second lens does not exceed the lensdiameter. The second drive with correspondingly small bore diameter canbe disposed in this region. Furthermore, the optimum lens separation isapproximately the sum of the focal lengths. The sum of focal lengths,which is relatively large for a Keplerian telescope, provides additionalspace for the drives.

In an alternative implementation, the first lens of the secondtelescopic configuration is designed as convergent lens and the secondlens is designed as dispersive lens. The telescopic configuration is aGalilean telescope in this case, and the separation between the firstand second lenses may be chosen to be very small.

In a further implementation, the first lens and the second lens of thesecond telescopic configuration are moveable in the beam direction inopposite directions by approximately identical path lengths. Thedivergence of the beam can thereby be changed in a particularlyadvantageous manner. The lens displacement required to adjust the focusalong the beam axis is divided to two lenses, thereby halving therequired lens displacement for each of the two lenses, whichconsiderably increases the dynamics of the lens motion. Since the motionof the lenses is symmetrically opposite, it is moreover ensured thatonly small reaction forces act on the mounting of the focusing device.

In another implementation, the first lens and/or the second lens of thesecond telescopic configuration is/are disposed in a bearing, whichincludes at least one annular membrane spring whose inner diametercorresponds substantially to the lens diameter. The membrane springincludes, in particular, a plurality of azimuthal slots.

The bearing may include two membrane springs that are spaced apart fromeach other in the beam direction and are connected to each other in theregion of the inner diameter by a tube extending parallel to the beamdirection. The large radial stiffness of the individual membrane therebyprovides the composite with a high dumping resistance (principle ofparallelogram bearing).

In another general aspect, a scanner processing head forthree-dimensional workpiece processing includes the above-describeddevice for focusing the laser beam. The position of the focal point canbe adjusted in a first and second spatial direction with scanner optics,for example, with one single mirror or with two mirrors, and in a thirdspatial direction through changing the focal position using the device.With such a scanner, the workpiece can be laser-processed in all threespatial directions with approximately the same speed.

In another general aspect, a method of focusing a divergent laser beamexiting from the exit plane of a laser light guide onto a workpieceincludes directing the laser beam through the first lens, directing thebeam from the first lens through the second lens, collimating the beamfrom the second lens with the collimating optical element, and focusingthe beam from the collimating optical element at the focal point usingthe focusing optical element. The method also includes establishing astandard distance between the first and second lenses such that themagnification of the exit plane of the laser light guide onto theworkpiece remains substantially constant when the lenses are displacedin opposite directions along the beam direction in order to move thefocal point along the beam direction.

The use of the device for focusing a laser beam is, however, not limitedto the application in a scanner processing head. It may be used, forexample, for deep welding processes to integrate an inventive telescopicconfiguration in a processing head without scanner optics for rapidadjustment of the focal position in the beam axis.

Further advantages can be extracted from the description and thedrawings. The features mentioned above and below may be used eitherindividually or collectively in arbitrary combination. Theimplementations shown and described are not to be understood asexhaustive enumeration but have exemplary character for describing thedevice. Other features will be apparent from the description, thedrawings, and the claims.

DESCRIPTION OF DRAWINGS

FIGS. 1 a-1 c show three partial views a, b, c of a device includingfirst and second telescopic configurations for focusing a laser beam tothree different focal points;

FIG. 2 shows a highly schematized longitudinal section of the secondtelescopic configuration of FIG. 1 with two bearings each including twoannular membrane springs;

FIG. 3 shows a top view of one of the annular membrane springs of FIG. 2with azimuthal slots; and

FIG. 4 shows a scanner processing head for three-dimensional materialprocessing with an inventive focusing device in accordance with FIG. 2.

Like reference symbols in the various drawings may indicate likeelements.

DETAILED DESCRIPTION

Referring to FIG. 1 a, a device 1 is used for focusing a laser beam 2that defines a beam direction 10. The device 1 serves to change a focalpoint of the laser beam 2 on a workpiece (not shown). The device 1includes a first telescopic configuration 3 including a collimating lens4 and a focusing lens 5 (diameter D) for focusing the laser beam 2 ontoa focal point 6. The device 1 includes a second telescopic configuration7 with a first lens 8 and a downstream second lens 9 (diameter d<D) thatare disposed in the beam path upstream of the first telescopicconfiguration 3. The first lens 8 and the second lens 9 of the secondtelescopic configuration 7 can be displaced relative to each other inthe beam direction 10. The second telescopic configuration 7 is disposedin the vicinity of an exit plane 11 of a laser light guide 12. Thesecond telescopic configuration 7 is designed as a Keplerian telescopesince the first lens 8 and the second lens 9 of the second telescopicconfiguration 7 are collimating lenses. Both lenses 8, 9 have the sameconstruction and therefore the same focal lengths. Altogether, oneobtains an optimum focal length range of approximately 20-60 mm for thelenses 8, 9 of the Keplerian telescope for the high-performance range ofa laser.

In another implementation, the second telescopic configuration 7 isdesigned as a Galilean telescope. In such a configuration, the focallength range tends to be slightly higher.

The function of the device 1 is exemplarily shown in the partial viewsshown in FIGS. 1 a through 1 c. FIG. 1 a shows the device 1 in the stateof rest. The first lens 8 and the second lens 9 of the second telescopicconfiguration 7 are disposed at a separation L from each other. Theseparation L is substantially the sum of the absolute values of focallengths of the first and second lenses 8, 9. In this state of rest, thefocal point 6 of the laser beam 2 is in plane B.

In FIG. 1 b, the first lens 8 and the second lens 9 are moved apart fromeach other in opposite directions along approximately identical pathlengths. In this state, the focal point 6 is displaced from the plane Bin the beam path behind plane B.

In FIG. 1 c, the first lens 8 and the second lens 9 are moved towardseach other along approximately identical displacement path lengths. Inthis state, the focal point 6 is displaced from the plane B in the beampath in front of plane B.

The separation L in the rest position is selected such that themagnification of the exit plane of the laser light guide onto theworkpiece remains substantially constant. The minimum variation in scalethat can be achieved depends on the selected optical configuration (forexample, the type and size of the telescopic configuration 3 or 7,length of the collimated beam, etc.) and on the required displacement ofthe focal position relative to the focal length of the focusing opticalelement of the first telescopic configuration 3. A variation of a smallpercentage of the image is maximally allowable without compensation (forexample, adjustment of the processing speed or laser power) depending onthe application.

The diameters d of the first 8 and second lenses 9 of the secondtelescopic configuration 7 should be as small as possible to improve thedynamic properties of the device 1. Small lens apertures resulting fromshort focal lengths increase the risk of overheating mounts and beamguiding elements (covers, apertures, etc.). In this case, active coolingmay be required. Depending on the shape of the optical fiber connectoror the socket, one obtains a minimum working separation (and therefore aminimum focal length) for the first lens 8 from the exit plane 11 of thelaser light guide 12.

Referring to FIG. 2, a bearing of the second telescopic configuration 7is shown for supporting the first and second lenses 8, 9. A firstbearing unit 17 bears the first lens 8. The first bearing unit 17includes an annular membrane spring 13 in which the first lens 8 isdisposed, a further member spring, and a pipe 14. The inner diameter ofthe spring 13 is adjusted to the diameter d of the lens 8. The membranespring 13 is connected to the further membrane spring 15 through thepipe 14. The pipe 14 is connected to a linear drive (not shown) fordisplacing (double arrow 16) the first lens 8 in the beam direction 10.The linear drive can be designed as a plunger drive and disposed in thebeam propagating direction 10 behind the second membrane spring 15coaxially to the laser beam.

The second telescopic configuration 7 includes a second bearing unit 18for bearing the second lens 9. Because the second lens 9 has the sameconstruction as the first lens 8, the second bearing unit 18 can havethe same construction as the first bearing unit 17. The overall lengthof the linear drives of the Keplerian telescope should maximally beapproximately four times the focal length.

FIG. 3 shows a top view of the membrane spring 13 of the secondtelescopic configuration 7. The membrane spring 13 preferably is made ofstainless steel, hardened spring steel, or phosphorus bronze, and has avery small stiffness in the axial direction, which has an advantageouseffect on the dynamics of the motion and on the required driving power.The membrane spring 13 is extremely rigid in the radial and azimuthaldirections such that a lens can move only in an axial direction butcannot be twisted or be laterally displaced. The membrane spring 13comprises a number of azimuthal slots 19 that can be produced forexample, through laser cutting and that strongly reduce the axialstiffness of the membrane spring 13 but still keep a high radialstiffness.

Referring to FIG. 4, the device 1 for focusing a laser beam 2 can beutilized with particular preference in a scanner processing head 20 forthree-dimensional workpiece processing. The position of the focal point6 can thereby be adjusted in a first and second spatial direction withtwo scanner mirrors 21, 22 and in a third spatial direction throughchanging the focal position using the device 1. The workpiece canthereby be processed in three dimensions, wherein the short lens shiftand the low mass of the lenses 8, 9 of the device 1 also ensureprocessing in the third spatial direction at a sufficiently high speed,since the focal point 6 can be quickly adjusted. The bearing with themembrane springs 13, 15 moreover ensures precise setting of the focalposition in the third spatial direction. The focal position can beadjusted in a controlled manner by measuring the deflection of thelenses.

Other implementations are within the scope of the following claims.

1. A scanner processing head for processing a three-dimensionalworkpiece, the scanner processing head comprising: a device for focusinga laser beam, the device comprising: a first telescopic configurationincluding a collimating optical element that collimates a laser beam,and a downstream focusing optical element that focuses the laser beam ona focal point; and a second telescopic configuration including a firstlens and a downstream second lens moveable relative to each other, thesecond telescopic configuration being disposed in a divergent beam pathupstream of the first telescoping configuration; and two scanner mirrorsoperable to adjust a position of the focal point in a first and secondspatial direction; wherein the device is operable to adjust a positionof the focal point in a third spatial direction.
 2. The scannerprocessing head of claim 1, wherein diameters of the first lens and thesecond lens of the second telescopic configuration are smaller thandiameters of the collimating optical elements of the first telescopicconfiguration.
 3. The scanner processing head of claim 1, wherein thefirst lens and the second lens of the second telescopic configurationeach have approximately the same absolute value of focal length.
 4. Thescanner processing head of claim 1, wherein the second telescopicconfiguration is disposed in the vicinity of an exit plane of a laserlight guide.
 5. The scanner processing head of claim 1, wherein aseparation between the first lens and the second lens of the secondtelescopic configuration is substantially equal to the sum of theabsolute values of their focal lengths.
 6. The scanner processing headof claim 1, wherein the first lens and the second lens of the secondtelescopic configuration are convergent lenses.
 7. The scannerprocessing head of claim 1, wherein the first lens of the secondtelescopic configuration is a convergent lens and the second lens is adispersive lens.
 8. The scanner processing head of claim 1, wherein thefirst lens and the second lens of the second telescopic configurationare moveable in opposite directions along the beam by approximatelyidentical path lengths.
 9. The scanner processing head of claim 1,wherein at least one of the first lens and the second lens of the secondtelescopic configuration is disposed in a bearing that comprises atleast one annular membrane spring having an inner diameter thatcorresponds substantially to a diameter of the at least one first orsecond lens.